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whence by substitution,

Apfendix I.
Ft = /, J*

or, a shear stress produces two normal stresses, tensile and corn-
pressive respectively, each of an equal intensity with its own, and
having directions at 45° to the original stress,

•^ 3&5-   Influence   of  Time   on   the   Stress-strain
Diagram.—The diagram in Fig. 743, taken by Professor Ewing,


of Tjurve

cf. 743.


shews very clearly the different plastic lines obtained, according
to whether the test experiment be made very quickly or very
slowly. It is clearly important that an average rate be maintained
in applying the straining load.

P. jgo. Wohler's Experiments.—To give a further
interest to Wohler's important experiments, the three machines
which he used are shewn in Figs. 744, 745, and 746. In the
first an axle is loaded by a spring at a considerable distance
* over-neck/ and is then rotated several millions of times before
breaking, the case being that of alternate stresses. The second
machine, Fig. 745, represents the bending of a beam, The load
is again caused by a spring in tension, and the varying stress is
obtained by the rotation of the lever B, placed below. This is the
case of a live load (removed and replaced). The third drawing,